several DMMs: set DC flag for diode mode

several DMMs: set DC flag for diode mode

Few DMM drivers already did it. This commit adjusts the remaining DMM
drivers, to set the "DC" flag for measurements in diode mode.

This fixes bug #144.

Although I don't have the hardware to test, the nature of the change and
the arrangement of driver code suggests it's good. When a meter already
communicated the "DC" status, the change does nothing and won't harm.
The change ensures "DC" is flagged for those meters which previously
didn't, which is desirable.